It is difficult to judge what impact Arrivabene has made in terms of engineering, Ferrari could be in the same position as they are now regardless of who took over as principal.
I think what you can say is though he has sparked life back into the people at Ferrari, comparing him to Domenicali is like chalk and cheese.
He's likely had almost no direct influence. The guy whose name I forgot who got replaced afaik was involved in bringing in the two key engine guys, he also was directly involved in calling Alonso's bluff and going behind his back to sign Vettel for the team and basically gave Alonso no choice but to move on. Multiple people in the team say the atmosphere has improved and everyone is happier about it.
So I think the last guy put most of the pieces in place, main reason Arrivabene as brought in was honestly Bernie. He is a friend of Bernie, has worked with him for years and knows how to deal with him while the last guy had a adversarial relationship with Bernie... which is not good for getting the best out of negotiations for your team.
They did it in the 80's as well, or early 90's, forget which. They fired a team principle who was failing, had a outspoken driver who was publicly criticising the car/team. They fired the team principle for not making a good enough car then brought in a guy for the rest of the year who pushed out the driver, made some big changes, but then he was replaced at the end of the year with someone who they saw as a more long term leader with relationships with key people up and down the grid and Bernie/others.
Basically Ferrari on both occasions brought someone to clean house, take all the heat you get when firing people's friends and bringing in outsiders. Then get rid of that guy and bring in a friendly, charismatic leader who also has long relationships with the key players in F1 for the influence that brings.
One guy to bring in the right people, a different nicer guy to coax the best out of them.
The extra hundred million ferrari has dumped into the engine department has probably helped a bit too!
I think it's just 100mil more for the team in general, I think that has been spread around quite a bit.